1. Field of the Invention
The present invention relates to a vibrating gyroscope including piezoelectric vibrators.
2. Description of the Related Art
Many digital video cameras and digital still cameras include an angular velocity sensor capable of detecting camera shake in order to correct motion blur caused by a shaking hand.
Such an angular velocity sensor drives piezoelectric vibrators, detects an electromotive voltage generated by vibrations of the piezoelectric vibrators caused by Coriolis force, and outputs a voltage signal corresponding to an angular velocity.
To correct motion blur caused by hand shake as described above, it is necessary to detect an angular velocity about a vertical axis (horizontal vibrations) and an angular velocity about a horizontal axis (vertical vibrations). To do this, a single sensor must have two detection axes perpendicular or substantially perpendicular to each other.
Conventionally, a two-axis vibrating gyroscope includes two sets each including a tuning-fork piezoelectric vibrator, a drive circuit (oscillation unit), and a signal processing circuit (detection unit), so that self-oscillation driving is performed independently for each axis.
When the two tuning-fork piezoelectric vibrators, which perform mechanical in-plane vibrations, are mounted on a single module, a difference in frequency between the two oscillation frequencies affects the oscillation frequency of each of the tuning-fork piezoelectric vibrators. This is due to mechanical propagation, aerial propagation, electrical propagation, and other characteristics of the vibrations. When a signal on which a difference frequency component is superimposed is synchronously detected by a detection circuit for each axis, the difference in frequency component appears as an interference wave in the output of the vibrating gyroscope. For example, when oscillation frequencies for first and second axes are about 25 kHz and about 31 kHz, respectively, a difference in frequency of about 6 kHz appears as an interference wave in the gyroscope output for each axis.
FIG. 1A and FIG. 1B illustrate examples of the interference described above. In the graph shown in FIG. 1A, the horizontal axis represents frequency and the vertical axis represents the level of an output signal of a vibrating gyroscope. In the example of FIG. 1A, an interference signal having a difference in frequency of about 6 kHz appears. In the graph shown in FIG. 1B, the horizontal axis represents time and the vertical axis represents the output signal level of the vibrating gyroscope for each axis. In the example of FIG. 1B, fluctuation components observed when a difference in oscillation frequency is about 11 Hz appear in output signals of the vibrating gyroscope for the two axes.
A camera in which the vibrating gyroscope is mounted amplifies the output signal of the vibrating gyroscope, converts the amplified signal into a discrete value through an analog-to-digital (AD) converter, sends the discrete value to a microcomputer, and calculates the amount of correction of motion blur caused by hand shake. If the level of noise resulting from an interference wave component is high enough to deviate from the resolution of the AD converter, erroneous detection of an angular velocity may occur.
Examples of a method for reducing such interference noise include a method in which oscillation frequencies of two vibrators are widely separated from each other, and a method in which an interference wave is attenuated by providing a filter.
However, the angular velocity sensitivity of a vibrator is inversely proportional to a difference in oscillation frequency. For example, if a difference in frequency between two axes of the vibrators is about 5 kHz, a sensitivity difference of about 25% will result. Therefore, when the oscillation frequencies of the two vibrators are widely separated from each other, a considerable difference in sensitivity of the vibrating gyroscope is observed between the two axes. As a result, a signal-to-noise (S/N) ratio of the piezoelectric vibrator having a higher oscillation frequency is degraded.
When a filter for attenuating an interference wave is providing, the frequency of the interference wave (i.e., a difference in frequency between two axes) is about 1/5 to about 1/15 of the oscillation frequencies for the two axes. Therefore, for example, it is necessary to provide several stages of low-pass filters (LPFs) having a cutoff frequency fc as low as about 300 Hz. However, insertion of stacked low-pass filters may affect the primary sensitivity range (from direct current (DC) to about 50 Hz) of the vibrating gyroscope, degrade the response characteristics of the vibrating gyroscope, and cause an increase in phase delay with respect to an applied angular velocity.
FIG. 2 is a graph showing an example of different response characteristics of a vibrating gyroscope depending on the presence or absence of a low-pass filter. In the graph of FIG. 2, the horizontal axis represents the frequency of angular velocity, that is, the frequency of an input signal to the low-pass filter, while the vertical axis represents the phase delay. In the example shown in FIG. 2, by providing the low-pass filter, a phase delay at a frequency of about 30 Hz increases by about 5 degrees.
Japanese Patent No. 3698787 describes a multi-axis vibrating gyroscope including a plurality of piezoelectric vibrators, an oscillation drive circuit, and a detection circuit. The vibrating gyroscope includes one oscillation circuit arranged to cause a piezoelectric vibrator for a first axis to self-oscillate. Another piezoelectric vibrator for at least one remaining axis is caused to oscillate under excitation using an oscillation signal for the first axis.
FIG. 3 is a block diagram illustrating an example of a drive detection unit included in the vibrating gyroscope described in Japanese Patent No. 3698787. This vibrating gyroscope includes a drive detection circuit 41 and a detection circuit 42, which are respectively connected to two different piezoelectric vibrators (first and second piezoelectric vibrators) arranged with a space therebetween.
The drive detection circuit 41 includes an oscillation circuit 41a, a differential circuit 41b, a synchronous detection circuit 41c, and a rectifier circuit 41d. The detection circuit 42 includes a differential circuit 42a, a synchronous detection circuit 42b, and a rectifier circuit 42c. 
The first piezoelectric vibrator is connected to terminals 7, 8, 9, and 10 of the drive detection circuit 41, while the second piezoelectric vibrator is connected to terminals 7′, 8′, 9′, and 10′ of the detection circuit 42.
In the drive detection circuit 41, the oscillation circuit 41a is connected to detection electrodes of the first piezoelectric vibrator, a drive electrode of the first piezoelectric vibrator, and the synchronous detection circuit 41c so as to define a self-oscillation circuit. At the same time, the oscillation circuit 41a is connected to a drive electrode of the second piezoelectric vibrator and the synchronous detection circuit 42b. With this configuration, an oscillation signal from the oscillation circuit 41a is supplied as a drive signal to the first and second piezoelectric vibrators, which are thus driven.
A detection signal from the first piezoelectric vibrator is supplied to the differential circuit 41b and differentially amplified.
The oscillation signal from the oscillation circuit 41a is supplied as a synchronous detection signal to the synchronous detection circuit 41c. The synchronous detection circuit 41c detects the differentially amplified signal in synchronization with the synchronous detection signal, and outputs the differentially amplified signal as a detection signal. The detection signal is rectified by the rectifier circuit 41d and output from an output terminal 41e as a first detection voltage signal.
Similarly, a detection signal from the second piezoelectric vibrator is supplied through the terminals 8′ and 9′ to the differential circuit 42a. The synchronous detection circuit 42b detects the output of the differential circuit 42a in synchronization with the synchronous detection signal, and outputs the output of the differential circuit 42a as a detection signal. The detection signal is rectified by the rectifier circuit 42c and output from an output terminal 42d as a second detection voltage signal.
Japanese Unexamined Patent Application Publication No. 2007-285977 describes a multi-axis angular velocity sensor including a plurality of tuning-fork vibrators. In such an angular velocity sensor including two tuning-fork vibrators, the drive frequencies (oscillation frequencies) of the two tuning-fork vibrators are separated such that a difference in resonance frequency between the two tuning-fork vibrators (f1−f2) is at least about 2% of an average favg of the resonance frequencies of the two tuning-fork vibrators. Thus, by separating the oscillation frequencies of the two tuning-fork vibrators by at least a specified value, interference noise between circuits including the two tuning-fork vibrators is reduced.
Japanese Unexamined Patent Application Publication No. 2006-322874 describes a multi-axis vibrating gyroscope including a plurality of tuning-fork vibrators. In the vibrating gyroscope, only arm lengths of each of the tuning-fork vibrators are varied such that a difference in oscillation frequency between the tuning-fork vibrators is adjusted to be at least about 1 kHz. Thus, interference noise between axes is reduced.
With the configuration described in Japanese Patent No. 3698787, since the first piezoelectric vibrator is driven by self-oscillation, it is easy to obtain gyroscope characteristics that match the resonance frequency of the first piezoelectric vibrator. However, since the second piezoelectric vibrator is driven by excitation, and is thus excited at a non-resonance point, it is difficult to obtain original gyroscope characteristics. Moreover, since the first and second piezoelectric vibrators differ considerably from each other in gyroscope characteristics, a large amount of characteristic correction must be performed by appropriate circuits.
With the configurations described in Japanese Unexamined Patent Application Publications Nos. 2007-285977 and 2006-322874, even when the resonance frequencies of the tuning-fork vibrators are separated from each other, interference noise is generated to some extent. Moreover, as a difference in resonance frequency between the tuning-fork vibrators increases, a difference in gyroscope sensitivity between axes increases.
As described in Japanese Unexamined Patent Application Publication No. 2006-322874, when a low-pass filter is provided on a detection circuit side, interference noise is attenuated as a cutoff frequency of the low-pass filter is reduced. However, at the same time, this causes degradation in response characteristics of the vibrating gyroscope (i.e., delay of a sensor output signal with respect to an angular velocity).